Electronic stringed musical instruments such as guitar and bass often have a wide dynamic range, typically greater than 60 decibels. Such widely varying signals when passed through an amplifier circuit can cause problems, particularly when low voltage or battery operated electronic circuits are used to process the signals. In order to maintain high average signal levels through these circuits, dynamic range control is required.
Without some form of dynamic range control for electric music instrument amplifiers, typical active circuit components such as operational amplifiers and transistors can clip signal peaks and cause undesirable distortion. Distortion created by this type of clipping typically contains high levels of odd order harmonics (i.e., 3rd,5th, 7th . . . ,), which are considered by many to be harsh sounding and generally unacceptable if excessive levels are allowed.
One known method of dynamic range control is to use simple voltage dividing circuits or variable resistors to adjust incoming signals so that the maximum levels expected do not exceed certain predetermined limits. These circuits are usually adjusted only once. However, much of the available headroom is only used during signal transients. This results in low average signal levels and overall signal to noise ratios that are less than desirable.
Another method is to limit peak signal excursions by using diodes or transistors placed in the circuit in such a way that when signal peaks reach a certain level signals are limited from going any higher. This limiting action keeps signal peaks away from amplifier nonlinearities and clipping levels. However, such limiting action can be abrupt and create high levels of odd order harmonic components, and an acceptable level of dynamic range control without excessive distortion may be difficult to achieve in practice.
Automatic gain control (AGC) circuits are also common in the art for use with electric musical instrument amplifiers. Such AGC circuits use variable gain and level sensing circuits that set circuit gain according to signal level. Such circuits can be effective and solve the problem of low average signal levels of the previous methods, but because much of the uniqueness of different musical instrument sounds is contained within the transients and peaks and valleys of the signal produced by these instruments, the individuality of instrument and player tends to be eliminated by using AGC devices, thereby tending to make many instruments sound somewhat the same. Furthermore, the basic AGC circuit building blocks can be relatively complex and costly. AGC circuits also usually have fixed attach and release times which further restricts the individual tonal quality of the instrument.
It is also known to use nonlinear circuits to introduce a controlled amount of distortion for obtaining a desired harmonic content as discussed in two publications by Richard A. Shaeffer entitled "Electronic Musical Tone Production by Nonlinear Waveshaping" in Volume 18, Number 4 of the Journal Of The Audio Engineering Society at pp. 413-416 (1970) and "Production of Harmonics and Distortion in p-n Junctions" in Volume 19, Number 9 of the Journal Of The Audio Engineering Society at pp. 759-768 (1971).
Other examples of peak limiting, clamping, and compression circuits are generally shown in U.S. Pat. Nos. 3,509,373; 3,986,049; 3,548,323; 4,119,922; and 4,349,788.